The following publications are representative of the most relevant prior art known to Applicants at the time of the filing of this application.
______________________________________ U.S. Pat. Nos. ______________________________________ 4,070,197 January 24, 1978 Coes 4,419,161 December 6, 1983 Hailey 4,526,649 July 2, 1985 Gupta et al. ______________________________________
______________________________________ Other Publications ______________________________________ GB 2,015,910 September 19, 1979 Carborundum Co. GB 2,022,490 December 19, 1979 Olson GB 2,137,975A October 17, 1984 Kennedy et al. Iseki et al., J.Mat.Sci 15:1049 (1980) ______________________________________
Silicon carbide molded bodies having complex shapes are in practice made of several parts which have been joined together. However, in view of the relatively inert nature of silicon carbide, there has been considerable difficulty in effecting the joining operation, particularly when the end use of the joined silicon carbide pieces will be at temperatures of greater than 1500 degrees C. This is particularly true for beta-silicon carbide. Accordingly the art is replete with various methods for joining silicon carbide pieces. However, none of the prior art joining methods has been found sufficient to bond porous silicon carbide bodies to each other to produce a composite structure in which, after sintering, the joint is able to withstand temperatures of at least 1500.degree. C. without failure under a tensile load.
The joining of alpha-silicon carbide parts has been performed in the past generally either by (i) the use of adhesives or glues such as metal alloys, molten silicon, platinum pastes, borodiphenylsiloxane polymers in combination with silicon carbide powders, and the like or (ii) attempting in situ recrystallization of silicon carbide from silicon and carbon. Other methods which have been used include Coes (U.S. Pat. 4,070,197) teaches the use of a silicon carbide containing slip having the same bimodal alpha-silicon carbide composition as the parts being joined, but then after sintering must impregnate the composite structure with 10 to 30% silicon to actually form a bond which has strength Thus the use of a silicon carbide slip by itself was insufficient to form a strong bond. Moreover, the casting slip composition used failed to include any sintering aids, i.e. boron or carbon.
Hailey (U.S. Pat. 4,419,161) discloses joining either sintered or unsintered silicon carbide pieces by using metal borides, such as molybdenum boride (Mo.sub.2 B.sub.5), in a temporary binder which will leave little carbon char. When Hailey tried to use a cement of unsintered silicon carbide containing a carbon source and a sintering aid, he found that the "joints are not mechanically sound and are subject to breakage when exposed to mechanical shock" (Col. 5, 1. 2-6)
Another method of joining silicon carbide pieces is Gupta (U.S. Pat. 4,526,649) which teaches first roughening the silicon carbide surfaces to a depth of about 100-500 um by removal of excess silicon or by pitting and then filling the space with a cokable resin and adding liquid silicon to react with the resin at elevated temperatures in the absence of an applied force.
GB 2,015,910 teaches the use of a powdered cement comprised of molybdenum disilicide and a binder and heating to above 2030 C. in an inert atmosphere.
GB 2,022,490 joins pre-densified silicon carbide parts by first siliconizing them and inserting between them a cement of silicon carbide of the same particle size as the parts being joined in combination with carbon. Upon heating, formation of additional silicon carbide serves to join the parts.
GB 2,137,975A suggests putting a carbon source between the parts to be joined and then adding molten silicon to the joint to recrystallize silicon carbide to form a bond.
Iseki et al. joins dense sintered silicon carbide parts by placing a sinterable sub-micron silicon carbide powder containing boron and carbon as well as aluminum and iron between the parts and then subjecting the composite to hot pressing at a temperature of at least 1650 C. The parts so joined exhibit a substantial decrease in strength at temperatures above 1500 C.
Accordingly, there is a need for a method of joining porous silicon carbide bodies such that, after sintering, the joint is indistinguishable from the parts that have been joined and also does not become a failure point when the composite structure is subjected to a tensile load, particularly at temperatures of above 1500 C. For future silicon carbide applications, there is a need for a joining method which will produce a joint which, under scanning electron microscopic examination, is essentially indistinguishable from the pieces being joined. Thus it is an object of the present invention to produce a silicon carbide joint meeting these criteria.